The microchip industry commonly uses conventional plastic packages to protect microchips from the environment. For example, to protect its fragile microstructure, a die implementing a MEMS device may be secured within a pre-molded, plastic lead frame package. As another example, to protect its components from environmental contaminants, a die implementing an electronic circuit may be encapsulated within a post-molded, plastic lead frame package.
To those ends, the dies often are directly mounted to either or both the metal leads or metal die paddle of a lead frame within the package. This direct coupling between the die and lead frame, however, can create significant stress concentrations at the interface between the plastic/mold compound and the lead frame. For example, one widely used implementation packages a silicon die within a package having a copper lead frame. In that case, the differential between the coefficient of thermal expansion of the die and that of the lead frame is significant. Accordingly, temperature changes can cause damaging stresses between the lead frame and plastic/mold material of the package.
Undesirably, these stresses can pull and/or shear the plastic/mold material away from lead frame (i.e., causing delamination). Among other problems, this type of delamination can cause catastrophic wirebond lifting failures. Moreover, vibration forces generated by the dicing/sawing process also can cause the mold material to pull away from lead frame at the edge of the package.